The present invention relates to a semiconductor light emitting device and a method for producing the same, and more particularly, a semiconductor light emitting device comprising a gallium nitride type compound semiconductor for emission of blue light and a method for producing the same.
Such a gallium nitride type compound semiconductor is (1) a semiconductor comprising a compound of Ga of group III element and N of group V element, or (2) a semiconductor which comprises a GaN compound in which a part of Ga is substituted by other group III elements such as Al or In and/or a part of N is substituted by other group V elements such as P or As.
The semiconductor light emitting devices include light emitting diodes (hereinafter referred to as "LED") having pn junctions or double heterojunctions such as pin junctions, super-luminescent diodes (hereinafter referred to as "SLD"), and semiconductor laser diodes (hereinafter referred to as "LD").
Although conventional blue LEDs are lower in luminance than red or green ones and disadvantageous for practical use, they have been improved by using gallium nitride type compound semiconductor and more specifically, doping an amount of Mg thus forming a p-type semiconductor layer with a low resistance and are now available for new applications.
A conventional gallium nitride LED has a structure shown in FIG. 7. It is fabricated by applying gaseous forms of metal organic compounds such as trimethylgallium (TMG) and ammonia (NH.sub.3) together with a carrier of H.sub.2 gas to a single-crystal substrate 51 of sapphire (Al.sub.2 O.sub.3) at a low temperature of 400.degree. C. to 700.degree. C. using a metal organic chemical vapor deposition (MOCVD) method to form a low-temperature buffer layer 54 of, approximately 0.01 to 0.2 micrometer thick, comprising GaN, and applying the gaseous forms of the same materials at a high temperature of 700.degree. C. to 1200.degree. C. to form a high-temperature buffer layer 55, approximately 2 to 5 micrometers thick, comprising n-type GaN which is identical in the chemical composition to the layer 54.
A gaseous form of trimethylaluminum (TMA) is then added to the prescribed materials to deposit an n-type cladding layer 56 of, approximately 0.1 to 0.3 micrometer thick, comprising Al.sub.x Ga.sub.1-x N (where 0&lt;x&lt;1) for creating a double heterojunction. Those n-type layers are prepared by depending on the fact that gallium nitride type compound semiconductor materials can be made n-type without addition of any n-type impurities or by simultaneous application of SiH.sub.4 gas.
Then, the same materials including a less amount of Al and a more amount of In than in the cladding layers are deposited to form an active layer 57 which is comprising, for example, Ga.sub.y In.sub.1-y N (where 0&lt;y.ltoreq.1) and lower in band gap energy than the cladding layers.
Also, a p-type impurity of Mg or Zn in the form of a metal organic compound gas of e.g. bis(cyclopentadienyl)magnesium (CP.sub.2 Mg) or dimethylzinc (DMZn) is added to the same gaseous materials as of the n-type cladding layers in a reaction tube to form a p-type cladding layer 58 comprising p-type Al.sub.x Ga.sub.1-x N.
Furthermore, the same gaseous materials are applied for vapor deposition of a p-type GaN cap layer 59.
Whole surfaces of growth layers of the semiconductor material is then coated with a protective layer of e.g. SiO.sub.2 and the like and annealed for approximately 20 to 60 minutes at a temperature ranging from 400.degree. C. to 800.degree. C., allowing both the p-type cap layer 59 and the p-type cladding layer 58 to be activated. After the protective layer is removed, a resist pattern is applied for assigning n-type electrodes. When the semiconductor layers are subjected to dry etching by chlorine plasma atmosphere, desired regions of the n-type GaN high-temperature buffer layer 55 are exposed as shown in FIG. 7. Finally, two electrodes 61 and 60 are formed by sputtering of a metal film such as Au or Al. The semiconductor layers are then diced to LED chips.
As understood, a conventional semiconductor light emitting device using the gallium nitride type compound semiconductor material has at back side a sapphire substrate made of an insulating material. For forming electrodes on the back side, it is hence needed to use etching or other complicated processing method.
Although the sapphire substrate withstands a high temperature and is easily bonded to any type of crystal surface, the sapphire is very different from the gallium nitride semiconductor Al in lattice constant, 4.758 (sapphire substrate) angstrom to 3.189 (gallium nitride type semiconductor crystal) angstrom, and also, in coefficient of thermal expansion. The difference in lattice constant may result in crystal defect or dislocation in the buffer layer stacked on the sapphire substrate as denoted by A in FIG. 8. If the crystal defect propagates to the single-crystal gallium nitride type compound semiconductor layers which are stacked on the buffer layer and act as operating layers, operating region is declined and also optical characteristics of the semiconductor layers degrade.
In addition, the sapphire substrate is hardly cleft and it is thus not easy to produce semiconductor light emitting device chips by cleaving above-mentioned structure of the semiconductor layers. It is said that the conventional semiconductor layer structure described above is not appropriated for producing particular devices such as semiconductor laser devices in which two opposite sides are required to be mirror surfaces which are parallel with each other at high accuracy. It is also hard to process the sapphire substrate which may thus be processed with much difficulty.